Some of the existing methods used for flow visualization are oil flow, smoke, Schlieren, tufts, and neutrally buoyant helium bubbles. Each of these methods creates special problems. Oil is difficult to apply to the model in a continuous tunnel due to the velocity. It is messy and interpretation of the flow pattern is a vague art. The application of oil, to some extent, changes the geometry and, therefore, the performance of the model. Smoke disperses too rapidly at high tunnel velocities making it difficult to see the flow patterns, and the tunnel soon becomes saturated and must be vented. Smoke often contaminates by condensing on the model and tunnel surfaces and is difficult to remove. The smoke generator nozzle also produces some interference to the air flow upstream of the model. Schlieren, in general, only shows large temperature or pressure gradients and is complicated due to the necessary critical optical alignments. Tufts can only show direction of flow on a surface. Neutrally buoyant helium bubbles show streamlines but contaminate model and tunnel surfaces.